The present invention relates to a conductive paste exhibiting an excellent conductivity for a long period of time, and more particularly to a conductive paste which can form a conductive coating having an excellent conductivity for a long period of time through application of the conductive paste on a circuit board, such as a paper-phenolic resin board or a glass-epoxy resin board, by screen printing or the like, followed by heat curing. The conductive pulse is suitable for use in applications such as a conductor as a countermeasure against an electromagnetic wave noise of a circuit board or a conductor for wiring a circuit board.
Prior Art
In general, a conductive paste mainly comprises an organic binder, such as an epoxy resin, a saturated polyester resin, an acrylic resin or a phenolic resin (hereinafter simply referred to as the "binder"), a conductive powder and a solvent.
The above-described conductive paste has hitherto been used as a conductor for a circuit board. In recent years, an attempt has been made on the application of the conductive paste as an electromagnetic wave shielding material for a printed circuit board. Specifically, in this application, a printed circuit board comprises a board and formed thereon, a conductive layer having a circuit pattern including an earth pattern, wherein an insulated layer is formed on the surface of the board by means of printing to cover the conductive layer, except for an earth pattern portion of the surface of the board having a conductive layer formed thereon. A conductive paste is then applied thereto by means of printing to cover the insulating layer and connect with the earth pattern, thereby forming an electromagnetic wave shielding layer for use as a conductor of a circuit board for preventing an electromagnetic wave noise (see Japanese Patent Laid-Open No. 15497/ 1988 and Japanese Utility Model Laid-Open No. 29276/ 1980).
Attention has been drawn to conductive pastes, particularly a conductive copper paste as a conductor alternative to an expensive conductive silver paste. However, the conductive copper paste has the drawback that copper is more susceptible to oxidation than silver, so that although it is more inexpensive than the conductive silver paste, there arises a severe problem from a practical viewpoint as to the maintenance of the conductivity for a long period of time in the form of a paste or a heat-cured film.
Further, the conductive paste is generally insufficient in the adhesion to the metal or the insulating layer on the circuit board and therefore lacks in the reliability when used as a conductor of an electromagnetic wave shielding layer of a circuit board for preventing electromagnetic wave noise and a wiring conductor of a circuit board.
Various antioxidants and reducing agents have hitherto been added for the purpose of overcoming the above-described drawbacks. For example, addition of an anthracene derivative and an organic titanium compound to a conductive copper paste has been proposed for improving the prevention of oxidation. However, neither sufficient conductivity nor long-term stability of the conductivity has been attained as yet.
Examples of the method of improving the binder include a method wherein use is made of a melamine resin and a polyol and a polyester resin and/or an alkyd resin to improve the adhesion to the metal and the insulating layer (see Japanese Patent Laid-Open No. 253675/1987), a method wherein use is made of a mixture of a melamine resin with an acrylic resin to improve the adhesion to the metal (see Japanese Patent Laid-Open No. 83178/1988), and a method wherein use is made of poly-p-hydroxystyrene to improve the adhesion to the insulating layer as well as the releasability with the use of a solvent (see Japanese Patent Laid-Open No. 260663/1985). However, none of the above-described methods are satisfactory in the improving the adhesion, so that the adhesion to the metal and the insulating layer cannot be ensured.